1. Field of the Invention
This invention relates to evacuated flat panel displays such as those of the field emission cathode and plasma types and, more particularly, to the formation of spacer support structures for such a display, the support structures being used to prevent implosion of a transparent face plate toward a parallel spaced-apart back plate when the space between the face plate and the back plate is hermetically sealed at the edges of the display to form a chamber, and the pressure within the chamber is less than that of the ambient atmospheric pressure. The invention also applies to products made by such process.
2. Description of Related Art
For more than half a century, the cathode ray tube (CRT) has been the principal device for displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Although liquid crystal displays (LCD's) are now used almost universally for laptop computers, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and battery life is still measured in hours rather than days. Power consumption for computers having a color LCD is even greater, and thus, operational times are shorter still, unless a heavier battery pack is incorporated into those machines. In addition, color screens tend to be far more costly than CRTs of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of cold, pointed, field emission cathodes in combination with a phosphor-luminescent screen.
Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. Each triode has the following elements: a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed).
Although the phenomenon of field emission was discovered in the 1950's, only within the past ten years has research and development been directed at commercializing the technology. As of this date, low-power, high-resolution, high-contrast, full-color flat panel displays with a diagonal measurement of about 15 centimeters have been manufactured using field emission cathode array technology. Although useful for such applications as viewfinder displays in video cameras, their small size makes them unsuited for use as computer display screens.
In order for proper display operation, which requires field emission of electrons from the cathodes and acceleration of those electrons to the screen, an operational voltage differential between the cathode array and the screen of at least 1,000 volts is required. As the voltage differential increases, so does the life of the phosphor coating on the screen. Phosphor coatings on screens degrade as they are bombarded by electrons. The rate of degradation is proportional to the rate of impact. As fewer electron impacts are required to achieve a given intensity level at higher voltage differentials, phosphor life will be extended by increasing the operational voltage differential. In order to prevent shorting between the cathode array and screen, as well as to achieve distortion-free image resolution and uniform brightness over the entire expanse of the screen, highly uniform spacing between the cathode array and the screen must be maintained. During tests performed at Micron Display Technology, Inc. in Boise, Id., it was determined that, for a particular evacuated, flat-panel field emission display utilizing glass support columns to maintain a separation of 250 microns (about 0.010 inches), electrical breakdown occured within a range of 1100-1400 volts. All other parameters remaining constant, breakdown voltage will rise as the separation between screen and cathode array is increased. However, maintaining uniform separation between the screen and the cathode array is complicated by the need to evacuate the cavity between the screen and the cathode array to a pressure of less than 10.sup.-6 torr so that the field emission cathodes will not experience rapid deterioration.
Small area displays (e.g. those which have a diagonal measurement of less than 3.0 cm) may be cantilevered from edge to edge, relying on the strength of a glass screen having a thickness of about 1.25 mm to maintain separation between screen and the cathode array without significant deflection in spite of the atmospheric load. However, as display size is increased, the weight of a cantilevered flat glass screen must increase exponentially. For example, a large rectangular television screen measuring 45.72 cm (18 in.) by 60.96 cm (24 in.) and having a diagonal measurement of 76.2 cm (30 in.), must support an atmospheric load of at least 28,149 newtons (6,350 lbs.) without significant deflection. A tempered glass screen or faceplate (as it is also called) having a thickness of at least 7.5 cm (about 3 inches) might well be required for such an application. But that is only half the problem. The cathode array structure must also withstand a like force without significant deflection. Although it is conceivable that a lighter screen could be manufactured so that it would have a slight curvature when not under stress, and be completely flat when subjected to a pressure differential, the fact that atmospheric pressure varies with altitude and as atmospheric conditions change makes such a solution impractical.
A more satisfactory solution to cantilevered screens and cantilevered cathode array structures is the use of closely spaced dielectric support structures (also referred to herein as load-bearing spacers) each of which bears against both the screen and the cathode array plate, thus maintaining the two plates at a uniform distance between one another, in spite of the pressure differential between the evacuated chamber between the plates and the outside atmosphere. Such a structure makes possible the manufacture of large area displays with little or no increase in the thickness of the cathode array plate and the screen plate. It is interesting to note that a single cylindrical quartz column having a diameter of 25 microns (0.001 in.) and a height of 200 microns (0.008 in.) a buckle load of about 2.67.times.10.sup.-2 newtons (0.006 lb.). Buckle loads are somewhat less if glass is substituted for quartz. Buckle loads also decrease as height is increased with no corresponding increase in diameter. It is also of note that a cylindrical column having a diameter d will have a buckle load that is only slightly greater than that of a column having of square cross section and a diagonal d. If quartz column support structures having a diameter of 25 microns and a height of 200 microns are to be used in the 76.2 cm diagonal display described above, slightly more than one million spacers will be required to support the atmospheric load. To provide an adequate safety margin that will tolerate foreseeable shock loads, that number would probably have to be doubled.
Load-bearing spacer support structures for field-emission cathode array displays must conform to certain parameters. The support structures must be sufficiently nonconductive to prevent catastrophic electrical breakdown between the cathode array and the anode (i.e., the screen). In addition to having sufficient mechanical strength to prevent the flat panel display from imploding under atmospheric pressure, they must also exhibit a high degree of dimensional stability under pressure. Furthermore, they must exhibit stability under electron bombardment, as electrons will be generated at each pixel location within the array. In addition, they must be capable of withstanding "bakeout" temperatures of about 400.degree. C. that are likely to be used to create the high vacuum between the screen and the cathode array back plate of the display. Also, the material from which the spacers are made must not have volatile components which will sublimate or otherwise outgas under low pressure conditions. For optimum screen resolution, the spacer support structures must be nearly perfectly aligned to array topography, and must be of sufficiently small cross-sectional area so as not to be visible. Cylindrical spacers support structures must have diameters no greater than about 50 microns (about 0.002 inch) if they are not to be readily visible.
There are a number of drawbacks associated with certain types of spacer support structures which have been proposed for use in field emission cathode array type displays. Support structures formed by screen or stencil printing techniques, as well as those formed from glass balls lack a sufficiently high aspect ratio. In other words, spacer support structures formed by these techniques must either be so thick that they interfere with display resolution, or so short that they provide inadequate panel separation for the applied voltage differential. A process of forming spacer support structures by masking and etching deposited dielectric layers in a reactive-ion or plasma environment to a depth of at least 250 microns suffers not only the problem of slow manufacturing throughput, but also that of mask degradation, which will result in the spacer support structures having non-uniform cross-sectional area throughout their lengths. Likewise, spacer support structures formed from lithographically defined photoactive organic compounds are totally unsuitable for the application, as they tend to deform under pressure and to volatize under both high-temperature and low-pressure conditions. Techniques which adhere stick shaped spacers to a matrix of adhesive dots deposited at appropriate locations on the cathode array back plate are typically unable to achieve sufficiently accurate alignment to prevent display resolution degradation, and any misaligned stick which is adhered to only the periphery of an adhesive dot may later become detached from the dot and fall on top of a group of nearby cathode emitters, thus blocking their emitted electrons.
What is needed is a new method of manufacturing dielectric, load-bearing spacer support structures for use in field emission cathode array type displays. The resulting support structures must have high aspect ratios, near-perfect alignment on both the screen and backplate, resist deformation under pressure and be compatible with very low pressure and high temperature conditions.